Porous membrane structure and method

ABSTRACT

A method of treating a membrane comprises the steps of providing a membrane with surfaces that define a plurality of pores extending through the membrane. Providing a dispersion of oleophobic fluoropolymer solids. Stabilizing the dispersion with a stabilizing agent. Diluting the dispersion with a wetting agent. Wetting surfaces which define the pores in the membrane with the diluted and stabilized dispersion. Removing the wetting agent and the stabilizing agent from the membrane. Coalescing the oleophobic fluoropolymer solids of the dispersion on surfaces that define pores in the membrane. A composite membrane comprises a porous membrane having a plurality of interconnecting pores extending through the membrane and made from a material which tends to absorb oils and certain contaminating surfactants. A coating is disposed on surfaces of the nodes and fibrils defining the interconnecting passages in the membrane. The coating comprises oleophobic fluoropolymer solids coalesced on surfaces of the nodes and fibrils to provide oil and surfactant resistance to the composite membrane without completely blocking pores in the membrane. The composite membrane is gas permeable, liquid penetration resistant and capable of moisture vapor transmission at a rate of at least 70,000 gr/m 2 day.

RELATED APPLICATIONS

The present application is a continuation-in-part application of U.S.patent application Ser. No. 09/458,301 filed Dec. 10, 1999, now U.S.Pat. No. 6,480,084, which is a divisional application of U.S. patentapplication Ser. No. 09/249,519 filed Feb. 12, 1999, now U.S. Pat. No.6,228,447.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to a composite membrane and amethod of making the composite membrane. In particular, the presentinvention relates to a porous membrane that is treated to provideoleophobic properties to the membrane and to a method of treating themembrane.

2. Description of the Prior Art

It is known that technical fabrics must be suitable for use in demandingapplications. Examples of such demanding applications include filterelements, outerwear garments and apparel, footwear, tents, sleepingbags, protective garments, clean room garments, surgical drapes,surgical gowns, other types of barrier wear and allergen barrierproducts. The technical fabrics often include a film or membrane toprotect the fabric user from an external condition or environment and/orprotect the external environment from contamination by the user. Thefilm or membrane may be made from any suitable material or structure andin any suitable manner.

A known material for the membrane that has proven particularly suitablefor such demanding applications is made of an expandedpolytetrafluoroethylene (“ePTFE”) material. The ePTFE membrane istypically microporous and laminated to at least one other material, suchas a textile base or shell fabric. The resulting membrane and fabriclaminate can be used to manufacture any number of finished products tomeet the demands of the particular application.

It is known that an ePTFE membrane is air permeable and moisture vaportransmissive, yet resistant to wind and liquid penetration at moderatepressures. However, the ePTFE membrane tends to absorb oils and certaincontaminating agents, such as body oils contained in perspiration, fattysubstances or detergent-like contaminants. When the ePTFE membranebecomes contaminated by absorbing the oils or other contaminatingagents, the membrane may no longer effectively resist liquidpenetration.

One known approach to rendering an ePTFE membrane resistant tocontamination by absorbing oils or contaminating agents includesapplying a layer of polyurethane onto, or partially into, the ePTFEmembrane, as disclosed in U.S. Pat. No. 4,194,041. A membrane with apolyurethane layer is generally contaminating agent resistant and hasrelatively high moisture vapor transmission rates. However, air may notfreely permeate through the polyurethane layer. It is known that somedegree of air permeability is desirable to increase user comfort. It isalso known that the polyurethane layer must be wet in order toeffectively transmit moisture vapor which can feel cold, wet and“clammy” to the user.

Another known approach to contamination resistance is to coat surfacesdefining the pores in a porous membrane with a fluoroacrylate monomerand a polymerization initiator, as disclosed in U.S. Pat. No. 5,156,780.The initiator is activated to polymerize the monomer in situ to coatsurfaces defining the pores in the membrane. This approach provides amembrane that is somewhat air permeable and resistant to absorbing oilsand contaminating agents. However, this approach requires relativelyexpensive equipment and materials, such as an ultraviolet curing stationand a nearly oxygen-free or inert atmosphere, to process and polymerizethe monomer once it is applied to the membrane. Furthermore, thisapproach requires solvents, cross-linking reactants and/or initiatorsthat may be environmentally unsound and difficult to obtain.

Yet another known approach is to coat a microporous membrane with anorganic polymer having recurring pendant fluorinated organic sidechains, as disclosed in U.S. Pat. No. 5,539,072. An aqueous dispersioncarries the polymer and cannot enter the pores of the membrane. Arelatively expensive fluorosurfactant is used in amounts that may bedifficult to completely remove from the membrane in order to permit thepolymer in the aqueous dispersion to enter the pores of the membrane.

Thus, a need exists to provide a membrane that is air permeable,moisture vapor transmissive, wind and liquid penetration resistant,durably resists absorbing oils and certain contaminating agents, isrelatively inexpensive and easy to manufacture, made from readilyavailable materials and does not require relatively expensive equipment,processes or materials. There is also a need to limit the agglomerationand “settling” of dispersed particles or “solids” in a coating that isto be applied to the membrane for a predetermined duration.

SUMMARY OF THE INVENTION

The present invention is directed to a method of treating a membrane.The method comprises the step of providing a membrane with surfaces thatdefine a plurality of pores extending through the membrane. A dispersionof oleophobic fluoropolymer solids is provided. The dispersion isstabilized with a stabilizing agent. The dispersion is diluted with awetting agent. Surfaces which define the pores in the membrane arewetted with the diluted and stabilized dispersion. The wetting andstabilizing agents are removed from the membrane. The oleophobicfluoropolymer solids of the dispersion are coalesced on surfaces thatdefine pores in the membrane.

The step of providing a membrane comprises providing a microporousmembrane, such as a membrane made from expanded polytetrafluoroethylene.The diluting step comprises diluting the stabilized dispersion in awetting agent so the diluted and stabilized dispersion has a surfacetension and contact angle relative to the membrane such that the dilutedand stabilized dispersion is capable of wetting surfaces defining thepores in the membrane. The step of stabilizing the dispersion comprisesproviding the stabilizing agent in a weight amount in the range of 0.5to 5 times the weight of the wetting agent. The step of stabilizing thedispersion includes providing water as the stabilizing agent.

The step of providing a dispersion of oleophobic fluoropolymer solidscomprises providing a dispersion of acrylic based polymer withfluorocarbon side chains. The step of diluting the dispersion ofoleophobic fluoropolymer solids with a wetting agent comprises providingthe wetting agent in a weight amount in the range of 2 to 10 times theweight of the dispersion. The coalescing step comprises heating theoleophobic fluoropolymer solids to a temperature in the range of 200° C.to 240° C. for at least ten seconds to flow and coalesce the oleophobicfluoropolymer solids on surfaces defining the pores in the membranewithout completely blocking the pores.

The present invention is also directed to a composite membrane. Thecomposite membrane comprises a porous membrane having a plurality ofinterconnecting pores extending through the membrane and made from amaterial that tends to absorb oils and certain contaminatingsurfactants. A coating is disposed on surfaces of the nodes and fibrilsdefining the interconnecting passages in the membrane. The coatingcomprises oleophobic fluoropolymer solids coalesced on surfaces of thenodes and fibrils to provide oil and surfactant resistance to theresultant composite membrane without completely blocking pores in themembrane. The membrane is gas permeable, liquid penetration resistantand capable of moisture vapor transmission at a rate of at least 70,000gr/m²day.

The composite membrane includes the membrane being made from expandedpolytetrafluoroethylene. The composite membrane is gas permeable at arate of at least 0.10 cubic feet per minute per square foot.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the present invention will become apparent to thoseskilled in the art to which the present invention relates from readingthe following description with reference to the accompanying drawings,in which:

FIG. 1 is a schematic sectional view of a laminated fabric that includesa composite membrane embodying the present invention;

FIG. 2 is an enlarged schematic plan view of a portion of the membraneillustrated in FIG. 1, viewed approximately along the line 2—2 in FIG.1;

FIG. 3 is a greatly enlarged schematic sectional view of a portion ofthe membrane in FIG. 2, illustrating a coating disposed on surfaces ofnodes and fibrils that define pores in the membrane;

FIG. 4 is a schematic illustration of the relationship between a liquiddrop and a solid;

FIG. 5 is a schematic view of equipment used in the method of coatingthe membrane according to the present invention;

FIG. 6 is an SEM photograph of a membrane prior to the application ofthe coating; and

FIG. 7 is an SEM photograph of a membrane after being coated accordingto the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Laminated fabric 10 (FIG. 1) incorporating a composite membrane 12, madeaccording to the present invention, is wind and liquid penetrationresistant, moisture vapor transmissive and air permeable. The laminatedfabric 10 is resistant to contamination by absorbing or adsorbing oilsand certain contaminating agents, such as body oils, fatty substances,detergent-like contaminants or perspiration that contains oil-basedcomponents. The laminated fabric 10 also includes a layer of textilebase or shell fabric material 14 that is laminated to the compositemembrane 12 by any suitable process. The shell fabric 14 may be madefrom any suitable material that meets performance and other criteriaestablished for a given application in which the laminated fabric 10will be used.

“Moisture vapor transmissive” is used to describe an article thatpermits the passage of water vapor through the article, such as thelaminated fabric 10 or composite membrane 12. The term “resistant toliquid penetration” is used to describe an article that is not “wet” or“wet out” by a challenge liquid, such as water, and prevents thepenetration of liquid through the membrane under ambient conditions ofrelatively low pressure. The term “resistant to wind penetration”describes the ability of an article to prevent air penetration abovemore than about three (3) CFM per square foot at a pressure differentialacross the article of 0.5″ of water. The term “oleophobic” is used todescribe an article that is resistant to contamination by absorbing oradsorbing oils, greases or body fluids, such as perspiration and certainother contaminating agents.

The composite membrane 12 embodying the present invention includes amembrane 16. The membrane 16 is porous, and preferably microporous, witha three-dimensional matrix or lattice type structure of numerous nodes22 (FIG. 2) interconnected by numerous fibrils 24. The material that themembrane 16 is made from is preferably expanded polytetrafluoroethylene(“ePTFE”). Surfaces of the nodes 22 and fibrils 24 define numerousinterconnecting pores 26 that extend through the membrane 16 betweenopposite major side surfaces 18, 20 (FIG. 1) of the membrane.

By way of example, garments or other finished products incorporating thelaminated fabric 10 permit moisture vapor transmission through thegarment. Moisture vapor typically results from perspiration. The garmentor finished product permits moisture vapor transmission at a ratesufficient for the user to remain dry and comfortable during use in mostconditions. The laminated fabric 10 is also resistant to liquid and windpenetration, while being air permeable. The membrane 16 has a tendencyto become contaminated by absorbing or adsorbing certain contaminatingmaterials such as oils, body oils in perspiration, fatty substances,detergent-like surfactants or other contaminating agents. When themembrane 16 becomes contaminated, resistance to liquid penetration maybe lost.

The membrane 16 of the present invention could be coated with anoleophobic fluoropolymer material in such a way that enhanced oleophobicand hydrophobic properties result without compromising its airpermeability or moisture vapor transmission rate. The membrane 16 has acoating 28 (FIG. 3), embodying the present invention, to provideincreased hydrophobic and oleophobic properties. The coating 28surrounds and adheres to the nodes 22 and fibrils 24 that define thepores 26 in the membrane 16. The coating 28 also conforms to thesurfaces of most, and preferably all, the nodes 22 and fibrils 24 thatdefine the pore 26 in the membrane 16. The coating 28 improves theoleophobicity of the membrane 16 by resisting contamination fromabsorbing or adsorbing materials such as oils, body oils inperspiration, fatty substances, detergent-like surfactants and othercontaminating agents. The composite membrane 12 embodying the presentinvention has durable liquid penetration resistant when subjected torubbing, touching, folding, flexing, abrasive contact or laundering.

Microporous membranes 16, such as those made from such as ePTFE, have amultiplicity of interconnecting voids which allow the transfer of fluids(gases or liquid vapors) from one outer major side membrane surface 18or 20 to the opposite outer membrane major side surface 20 or 18. Themembrane 16 may or may not resist the transfer of a given liquiddepending on the free energy properties of the given liquid and membranesurfaces. The surface energy of many microporous membranes, includingePTFE, is such that the membrane resists penetration by relatively highsurface tension liquids, such as water-based materials. Unfortunately,many undesirable liquids such as body oils have relatively low surfacetensions, and have a tendency to be absorbed or adsorbed into thematerial of the membrane, even those made of ePTFE. Because water basedmaterials normally have a relatively high surface tension, it is verydifficult to coat the surfaces of the fibers and nodes within themembrane with water-based materials to make them resistant to theabsorption or adsorption of fluids like body oils.

The present invention provides a process where water-based dispersionsare modified so they can enter the pores of a microporous membrane suchas ePTFE. The surface tension of the water-based dispersion can bereduced to where the water-based dispersion is drawn into the pores 26of the membrane 16 by capillary action displacing the air occupyingthese pores and coating all the fibers and nodes within the membrane 16.The resultant composite membrane 12 will then resist the absorption oradsorption of fluids like body oils but still allow the transfer of airand moisture in the form of vapor.

The concept of a liquid drop 40 (FIG. 4) wetting a solid material 42 isalso fundamental to understanding the present invention. The physicaland thermodynamic definition of “wetting” is based on the concepts ofsurface energy and surface tension. Liquid molecules are attracted toone another at their surfaces. This attraction tends to pull the liquidmolecules together. Relatively high values of surface tension mean thatthe molecules have a strong attraction to one another and it isrelatively more difficult to separate the molecules. The attractionvaries depending on the type of molecule. For example, water has arelatively high surface tension value because the attraction in watermolecules is relatively high due to hydrogen bonding. Fluorinatedpolymers or fluoropolymers have a relatively low surface tension valuebecause of the strong electronegativity of the fluorine atom.

A contact angle Ø is defined as the angle between the liquid drop 40 anda surface 44 of the solid 42 taken at the tangent edge of where theliquid drop contacts the solid surface. The contact angle is 180° when aliquid forms a spherical drop on the solid surface. The contact angle is0° when the drop spreads to a thin film over the solid surface.

The free energy between a solid and a liquid is inversely related to themolecular attraction between the solid and the liquid. The free energyof the solid relative to a liquid is often referred to as the surfaceenergy γ_(SL) of the solid relative to the liquid. The free energy ofliquid relative to air is normally called the surface tension of theliquid γ_(LA). The free energy of the solid relative to air is normallyreferred to as the surface energy of the solid γ_(SA). The Young-Dupreequation relates all the free energies to the contact angle as Ø:

γ_(SA)−γ_(SL)=γ_(LA)*Cos (Ø)  (Eq. 1)

The degree to which a challenge liquid may “wet” a challenged soliddepends on the contact angle Ø. At a contact angle Ø of 0°, the liquidwets the solid so completely that a thin liquid film is formed on thesolid. When the contact angle Ø is between 0° and 90° the liquid wetsthe solid and there is a degree of adhesion between the liquid and thesolid. When the contact angle Ø is more than 90° the liquid does not wetthe solid.

For example, consider two different liquids on a polytetrafluoroethylene(“PTFE”) solid surface that has a surface energy γ_(SA) of 19 dynes/cm.One liquid, such as isopropyl alcohol (“IPA”) has a surface tensionγ_(LA) of 22 dynes/cm (which is a higher value than the surface energyγ_(SA) value of the PTFE material and one might think cannot wet thePTFE material) and a relative contact angle Ø of about 43° relative toPTFE. Therefore, IPA “wets” PTFE very well. The γ_(SL) of IPA relativeto PTFE can now be calculated by rearranging Eq. 1 to:

γ_(SL)=γ_(SA)−γ_(LA)*Cos (Ø)

γ_(SL)=19−22*Cos (43°)=3 dynes/cm

In contrast, another liquid, such as deionized water has a surfacetension of about 72.1 dynes/cm at 77° F. and a contact angle Ø of 112°relative to PTFE and, therefore, does not wet PTFE or is “held out.” Thecalculated value for the surface energy γ_(SL) of water relative toPTFE, is 46 dynes/cm.

Another aspect of contact angle Ø is important. If the contact angle Øthat a given liquid makes relative to a solid is less than 90°, theliquid can be drawn into capillaries existing in even an apparentlysolid material. The amount of capillary force drawing the liquid intothe capillary will depend on the size of the capillary. A relativelysmaller capillary exerts a relatively greater force on the liquid todraw the liquid into the capillary. If the contact angle Ø is greaterthan 90°, there will be a force to drive the liquid out of thecapillaries. The capillary force relates to the surface energy γ_(SA) ofthe solid material and to the surface tension γ_(LA) of the liquid. Thecapillary force drawing the liquid into the capillaries increases withthe increasing surface energy γ_(SA) of the solid. The capillary forcedrawing the liquid into the capillaries also increases with decreasingsurface tension γ_(LA) of the liquid.

Likewise, if the contact angle Ø that a given liquid makes with a solidis greater than 90°, the liquid will be repelled out of the capillariesexisting in even an apparently solid material. The amount of capillaryforce drawing the liquid into the capillary will depend on the size ofthe capillary. A relatively smaller capillary exerts a relativelygreater force to expel the liquid from the capillary. The capillaryforce also relates to the surface energy γ_(SA) of the solid materialand to the surface tension γ_(LA) of the liquid. The capillary forceexpelling the liquid from the capillaries increases with the decreasingsurface energy γ_(SA) of the solid. The capillary force expelling theliquid from the capillaries also increases with increasing surfacetension γ_(LA) off the liquid.

The membrane 16 made from ePTFE contains many small interconnectedcapillary-like pores 26 (FIG. 2) that fluidly communicate withenvironments adjacent to the opposite major sides 18, 20 of themembrane. Therefore, the propensity of the ePTFE material of themembrane 16 to absorb or adsorb a challenge liquid, as well as whetheror not a challenge liquid would enter into the pores 26, is a functionof the surface energy γ_(SA) of the solid, the surface tension γ_(LA) ofthe challenge liquid, the relative contact angle Ø between the liquidand solid and the size or flow area of the capillary-like pores.

As described above, the present invention is concerned primarily with amicroporous ePTFE membrane 16. However, the present invention couldequally apply to any porous membrane made from a material that tends tobe oleophilic. Such membranes, when laminated to various shell fabrics,possess desirable liquid penetration resistant properties.Unfortunately, the ePTFE membrane 16 is susceptible to contamination byoils and certain contaminating agents, such as body oils, fattysubstances, detergent-like contaminants or perspiration that containsoil-based components. When the membrane 16 becomes contaminated, theresistance to liquid penetration may be reduced or lost.

Certain polymeric oleophobic coatings can impart a relatively lowsurface energy γ_(SA) to an ePTFE membrane so the relative contact angleØ of most challenge liquids, oils and contaminating agents is greaterthan 90° so they do not wet the membrane 16. There are several suchpolymeric oleophobic coatings that appear to be suitable. One example ofa suitable polymeric oleophobic coating is an oleophobic fluoropolymerwith acrylic-based polymer containing fluorocarbon side chains and ismarketed under the Zonyl® (a du Pont trademark) name.

Most of the oleophobic resins are made by emulsion and dispersionpolymerization and are sold as aqueous or water-based dispersions andinclude some surfactant to help suspend the solids in the dispersion.The oleophobic resins are typically used to treat fabrics as durablewater repellency (DWR) treatments for carpets as a dirt and stainresistant treatment. These treatments are used on fabric yarns, threads,filaments and fibers that are significantly larger in size than thenodes 22 and fibrils 24 of the membrane 16. These fabric yarns, threads,filaments and fibers normally have relatively high surface energies andare easily wet by water-based dispersions of oleophobic polymertreatments or known DWR treatments. These yarns, threads, filaments andfibers also define significantly larger voids even in a tightly knit orwoven fabric than the pores 26 in the membrane 16 so there is generallyno problem with coating all surfaces with the water-based dispersions ofoleophobic fluoropolymer treatments or known DWR treatments.

The contact angle Ø that these water-based dispersions of oleophobicfluoropolymer treatments or known DWR treatments make with certainmicroporous membranes, such as the ePTFE membrane 16 (and the surfacetension γ_(LA) of these water-based dispersions of oleophobicfluoropolymer treatments or known DWR treatments relative to the ePTFEmembrane) is such that the dispersions cannot wet the ePTFE membraneenough to be drawn into the pores 26 of the membrane. Consequently, theparticles or polymeric solids that are intended to coat the surfacesdefining the pores 26 in the membrane 16 do not contact those surfacesand may even completely block the pores of the membrane so the membraneis no longer air permeable. With many microporous membranes only onemajor side surface 18 or 20 of the membrane 16 can be coated usingwater-based dispersions of the known DWR treatments. The surfaces of thenodes 22 and fibrils 24 defining the pores 26 in the membrane 16 are notcoated and, thus, cannot provide the desired oleophobic properties tothe entire membrane.

In this case, there is no capillary repelling action from the pores 26within the membrane 16. Consequently, a contaminating materialphysically forced through the oleophobic coating and into the pores 26of the membrane 16 will significantly reduce the resistance to liquidpenetration of the membrane. In fact, if both surfaces 18, 20 of themembrane 16 are coated but the surfaces of the pores 26 are not, it ispossible for a contaminating material to be physically forced throughthe oleophobic coating and into pores where it will be locked into thesepores. This is because the oleophobic coating would create acapillary-barrier force for the contaminating material to overcome toget out of the inside pores 26. In contrast, if all the surfaces of thepores 26 are coated, as is possible with this invention, and acontaminate is forced into the pores, there would be a capillary forcetending to force the contaminate out of these pores. It is also likelythat any relatively small amount of coating that was able to attach to amajor side surface 18 or 20 of the membrane 16 is not very durable andcan be removed during use or laundering.

Substantially improved oleophobic properties of the microporous membrane16 can be realized if the surfaces defining the pores 26 in the membraneand the major side surfaces 18, 20 of the membrane are coated with awater-based dispersion of an oleophobic fluoropolymer. The limitingfactor has been the lack of an effective way to introduce thewater-based dispersion of the oleophobic fluoropolymer into the pores 26of the membrane 16 to coat the surfaces of the nodes 22 and fibrils 24that define the pores. The present invention provides a way to introducethe water-based dispersion of the oleophobic fluoropolymer into thepores 26 of the membrane 16 to coat the surfaces of the nodes 22 andfibrils 24 that define the pores without completely blocking the pores.

It has been found that the water-based dispersion of oleophobic resin orsolids is capable of wetting the membrane 16 and entering pores 26 in amicroporous membrane 16 when diluted by sufficient amount of awater-miscible wetting agent, for example IPA. The diluted dispersion ofoleophobic fluoropolymer has a surface tension γ_(LA) and relativecontact angle Ø that permit the diluted dispersion to wet and be drawninto the pores 26 of the membrane 16. The minimum amount of wettingagent that is required for the blend to enter the pores 26 in themembrane 16 depends on the surface tension γ_(LA) of the diluteddispersion required to obtain the required relative contact angle Øbetween the diluted dispersion and the material of the microporousmembrane 16 for drawing the dispersion into the pores of the membraneand to displace the air residing in the pores. This minimum amount ofwetting agent can be determined experimentally by adding drops ofdifferent blend ratios to the surface of the microporous membrane 16 andobserving which concentrations are immediately drawn into the pores 26of the membrane.

It has been found that liquid organic wetting agents tend to destabilizethe dispersions either by affecting the action of the surfactant or thedispersion particles, such as by softening the dispersion particles. Thedispersion particles or solids then have a tendency to attach to oneanother or “agglomerate”. This agglomeration yields a relatively largesolid that may be too large to enter a pore 26 in the membrane 16.Further, the agglomerated solids tend to “settle” out of the dispersionmaking it difficult to apply them to the membrane 16. The presentinvention provides a way to delay the removal of surfactant and theresultant agglomeration.

Liquid penetration resistance of a microporous membrane 16 may be lostif a challenge fluid or liquid can “wet” the membrane. The normallyhydrophobic microporous membrane 16 loses its liquid penetrationresistance when the liquid initially contacts and wets a major sidesurface 18 or 20 of the membrane and subsequently contacts and wets thesurfaces defining the pores 26 in the membrane. Progressive wetting ofthe surfaces defining the interconnecting pores 26 occurs until theopposite major side surface 20 or 18 of the microporous membrane 12 isreached by the wetting or “challenge” liquid. If the challenge liquidcannot wet the microporous membrane 16, liquid repellency is retained.

To prevent or minimize the loss of resistance to liquid penetration inan ePTFE membrane, the value of the surface energy γ_(SA) of themembrane must be lower than the value of the surface tension γ_(LA) ofthe challenge liquid and the relative contact angle Ø must be more than90°. Surface energy γ_(SA) and surface tension γ_(LA) values aretypically given in units of dynes/cm. Examples of surface energiesγ_(SA), surface tensions γ_(LA) and some measured contact angles Ø arelisted in the table below:

Surface Contact Material Energy Surface Tension Angle PTFE 19 dynes/cmtap water 77.5 dynes/cm 114°  deionized water   72 dynes/cm 112°  blood  60 dynes/cm perspiration   42 dynes/cm hexane 20.4 dynes/cm 25%Zonyl ® 7040 in IPA 25.3 dynes/cm 50° Zonyl ® 7040 polymer solids   4dynes/cm Zonyl ® 7040, as purchased 35.9 dynes/cm 79° Laundry detergentmix 30.9 dynes/cm Acetone 25.4 dynes/cm 37° 30 wt-% IPA 29.0 dynes/cm 40wt-% IPA 27.7 dynes/cm 50 wt-% IPA 26.8 dynes/cm 60 wt-% IPA 26.5dynes/cm 70 wt-% IPA 25.8 dynes/cm 43° 80 wt-% IPA 25.0 dynes/cm 90 wt-%IPA 24.5 dynes/cm 100 wt-% IPA  23.5 dynes/cm 24°

The more that the surface tension γ_(LA) of the challenge liquid isabove the surface energy γ_(SA) of the challenged material and/or themore the relative contact angle Ø is above 90°, the less likely thechallenge liquid will wet the challenged material.

The use of a coalesced oleophobic fluoropolymer solids from a suitablematerial, such as the water-based dispersion of Zonyl® 7040, to form thecoating 28 on the microporous membrane 16 reduces the surface energyγ_(SA) of the composite membrane 12 so fewer challenge liquids arecapable of wetting the composite membrane and enter the pores 26. Thecoalesced oleophobic fluoropolymer coating 28 of the composite membrane12 also increases the contact angle Ø for a challenge liquid relative tothe composite membrane. The oleophobic fluoropolymer solids from thewater-based dispersion of Zonyl® 7040 include an acrylic-based polymerwith fluorocarbon side chains. The side chains have been found to haveone of the lowest known surface tensions γ_(LA) so it is desirable tohave these extend away from the membrane 16. The oleophobicfluoropolymer solids used to coat the membrane 16 is preferably in theform of a stabilized water-miscible dispersion of perfluoroalkyl acryliccopolymer. The oleophobic fluoropolymer solids are dispersed primarilyin water, but may also contain relatively small amounts of acetone andethylene glycol or other water-miscible solvents and surfactants thatwere used in the polymerization reaction when the fluoropolymer solidswere made.

The coating 28 is disposed on and around surfaces of the nodes 22 andfibrils 24 that define the interconnecting pores 26 extending throughthe membrane 16. The coating 28 enhances the hydrophobic properties ofthe membrane 16 in addition to providing better oleophobic properties tothe membrane. It is contemplated that the coating 28 may be used totreat the membrane 16 only. However, the coating 28 may also be used totreat only the shell fabric 14 as durable water repellency (DWR)treatment in a separate process or at the same time the membrane 16 istreated if the shell fabric is laminated to the membrane.

The composite membrane 12 of the present invention has a relatively highmoisture vapor transmission rate (“MVTR”) and air permeability. Thecomposite membrane 12 has an MVTR, measured by a modified desiccantmethod, of at least 70,000 g/m²/24 hrs. The composite membrane 12 has anair permeability of at least 0.1 CFM per square foot.

The composite membrane 12 is air permeable to a sufficient degree andhas a sufficiently high MVTR that a user of the composite membrane canbe relatively comfortable in most conditions and even during periods ofphysical activity in wet conditions. Once a predetermined amount ofoleophobic fluoropolymer solids was properly coalesced on the membrane16, it was found that the pores 26 in the composite membrane 12 were notdramatically reduced in flow area from that of an uncoated membrane soair permeability and MVTR were significantly great to provide goodcomfort to a user of the composite membrane in articles such as jacketsand pants.

The membrane 16 is made by extruding a mixture of PTFE (available fromdu Pont under the name TEFLON®) fine particle resin and lubricant. Theextrudate is then calendered. The calendered extrudate is then“expanded” or stretched in machine and transverse directions to formfibrils 24 (FIG. 2) connecting the particles or nodes 22 in a threedimensional matrix or lattice type of structure, as illustrated in FIG.2. Surfaces of the nodes 22 and fibrils 24 define the plurality ofinterconnected pores 26 that are in fluid communication with one anotherand extend through the membrane 16 between opposite major sides 18, 20of the membrane. A suitable size for the pores in the membrane 16 may bein the range of about 0.2 to 10 microns, and is preferably in the rangeof about 1.0 to 5.0 microns. “Expanded” means sufficiently stretchedbeyond the elastic limit of the material to introduce permanent set orelongation to the fibrils 24. The membrane 16 may be fully sintered,partially sintered or unsintered. “Sintering” means changing the stateof the PTFE material from crystalline to amorphous.

Other materials and methods can be used to form a suitable microporous(defined here as having an average pore size of about 10 microns orless) membrane that has pores extending through the membrane. Forexample, other suitable materials that may be used to form a microporousmembrane include polyolefin, polyamide, polyester, polysulfone,polyether, acrylic and methacrylic polymers, polystyrene, polyurethane,polypropylene, polyethylene, cellulosic polymer and combinationsthereof.

After the ePTFE membrane 16 is manufactured, a stabilized and diluteddispersion of the oleophobic fluoropolymer solids is applied to themembrane to wet the surfaces of the nodes 22 and fibrils 24 that definethe pores 26 in the membrane. The thickness of the coating 28 and theamount and type of fluoropolymer solids in the coating may depend onseveral factors. These factors include the affinity of the solids toadhere and conform to the surfaces of the nodes 22 and fibrils 24 thatdefine the pores 26 in the membrane or whether abuse of the membraneduring use and laundering may crack, dislodge, damage or disrupt thecoating. After the wetting operation, substantially all of the surfacesof the nodes 22 and fibrils 24 are at least partially wetted, andpreferably all the surfaces of all the nodes and fibrils are completelywetted and the pores 26 in the membrane 16 are not blocked.

It is not necessary that the coating 28 completely encapsulate theentire surface of a node 22 or fibril 24 or be continuous to increaseoleophobicity of the membrane 16, but it is preferred. The finishedcoating 28 results from coalescing the oleophobic fluoropolymer solidson as many of the surfaces of the nodes 22 and fibrils 24 defining thepores 26 in the membrane 16 as possible. The preferred dispersion has asurface tension γ_(LA) that is greater than the surface energy γ_(SA) ofthe membrane 16 and/or a relative contact angle Ø that does not permitthe aqueous dispersion, as received, to wet the pores 26 in themembrane. The water-based dispersion is “stabilized” with a stabilizingagent, such as deionized and demineralized water, in order to delay theonset of agglomeration and settling of the oleophobic fluoropolymersolids. The stabilized dispersion is diluted in a water-miscible wettingagent, such as IPA. The diluted and stabilized dispersion has a surfacetension γ_(LA) and/or a relative contact angle Ø that permits thediluted and stabilized dispersion to enter the pores 26 in the membrane16 and wet the surfaces of the pores.

The oleophobic fluoropolymer solids of the diluted and stabilizeddispersion engage and adhere to surfaces of the nodes 22 and fibrils 24that define the pores 26 in the membrane 16 after the stabilizing andwetting agent materials are removed. The oleophobic fluoropolymer solidsare heated on the membrane 16 to flow and coalesce. The compositemembrane 12 is, thus, rendered resistant to contamination by absorbingor adsorbing oils and contaminating agents. During the application ofheat, the thermal mobility of the oleophobic fluoropolymer solids allowsthe solids to be mobile and flow around the nodes 22 and fibrils 24 andcoalesce to form a relatively thin and even coating 28. At therelatively elevated temperature, the mobility of the oleophobicfluoropolymer solids also permits the fluorocarbon side chains to orientthemselves to extend in a direction away from the nodes 22 and fibrils24. The coalesced oleophobic fluoropolymer provides a relatively thinprotective coating 28 on the membrane 16 that does not completely blockor “blind” the pores 26 in the composite membrane 12 which couldadversely affect moisture vapor transmission or air permeability throughthe composite membrane.

The preferred dispersion of oleophobic fluoropolymer solids includes anacrylic-based polymer with fluorocarbon side chains and a relativelysmall amount of water, water soluble co-solvent and glycol. There couldbe other solvents, co-solvents or surfactants in the aqueous dispersionwithout detracting from the spirit and scope of the present invention.One family of oleophobic fluoropolymer solids that has shown particularsuitability is the Zonyl® family of fluorine containing polymers (madeby du Pont and available from CIBA Specialty Chemicals). A particularlysuitable dispersion in the Zonyl® family is Zonyl® 7040. Othercommercially available chemicals that are suitable are TLF-8868,TLF-9312, TLF-9373, TLF-9404A and TLF-9494B all available from DuPont.These chemicals are typical examples of durable water repellency (“DWR”)treatments typically used for textiles, fibers and fabrics but notmicroporous membranes.

The dispersion of acrylic-based polymer with fluorocarbon side chains isstabilized with a stabilizing agent. A preferred stabilizing agent hasbeen found to be deionized and demineralized water. The stabilizingagent reduces the propensity of the oleophobic fluoropolymer solids fromsettling out and agglomerizing to a size which cannot enter a pore inthe membrane 16. The stabilized dispersion of acrylic-based polymer withfluorocarbon side chains is then diluted in a suitable wetting agent,such as ethanol, isopropyl alcohol, methanol, n-propanol, n-butanol,N-N-dimethylformamide, methyl ethyl ketone and water soluble e- and p-series glycol ethers.

A particularly suitable amount of oleophobic fluoropolymer solids in theZonyl® 7040 diluted and stabilized dispersion is in the range of about 5wt-% to 25 wt-%, and preferably 10 wt-% to 20 wt-%. The amount of IPApresent is in the range of about 30 wt-% to 90 wt-%, and preferably 60wt-% to 90 wt-%. The amount of stabilizing agent in the form ofdeionized and demineralized water is in the range of about 5 wt-% to 50wt-%, and preferably 15 wt-% to 25 wt-%. The diluted and stabilizeddispersion contains oleophobic fluoropolymer solids in the range ofabout 1.0 wt-% to about 5.0 wt-%, and preferably 2.0 to 4.0 wt-%. Theaverage particle size of the oleophobic fluoropolymer solids is about0.15 micron. The resulting diluted and stabilized dispersion has surfacetension γ_(LA) and a relative contact angle Ø properties that enable thediluted and stabilized dispersion to wet pores 26 in the membrane 16 andultimately be coated with oleophobic fluoropolymer solids. Thedispersion is diluted to provide a ratio by weight of wetting agent todispersion in the range of about 1:5 to 20:1, and preferably about 1.8:1to 5:1. The dispersion is stabilized by providing an amount ofstabilizing agent in the range of about 0.5 to 5 times the weight of thedispersion, and preferably an amount of stabilizing agent in the rangeof about 1 to 3 times the weight of the dispersion.

It should be apparent that the final solids content determines theamount of coating 28 that is applied to the membrane 16. The amount ofcoating 28 applied to the membrane 16 also depends on the coatingprocess. The combination of stabilizing agent and wetting agent is usedto dilute the “as purchased” dispersion to the desired solids content.It is desired that the ratio of stabilizing agent to wetting agent be amaximum in order to maximize the stability of the coating dispersion.However, enough wetting agent must be present to ensure proper wettingof the membrane 16 and flow of dispersion into the pores 26 in themembrane.

Method

Equipment 60 for use in the method of treating the membrane 16 accordingto the present invention is illustrated in FIG. 5. The method includesproviding the membrane 16 with surfaces defining a plurality of pores 26extending through the membrane. Preferably, the average size of thepores 26 in the membrane 16 is microporous. The membrane 16 preferablyis made from ePTFE.

The membrane 16, or alternatively the laminated fabric 10, is unreeledfrom a roll 62 and trained over rollers 64 and directed into a holdingtank or reservoir 66 over an immersion roller 68. A diluted andstabilized dispersion 80 of oleophobic fluoropolymer solids is in thereservoir 66. The dispersion is stabilized in a suitable stabilizingagent, such as deionized and demineralized water. The dispersion isstabilized by providing an amount of stabilizing agent in the range ofabout 0.5 to 5 times the weight of the dispersion, and preferably anamount of stabilizing agent in the range of about 1 to 3 times theweight of the dispersion. The stabilized dispersion of oleophobicfluoropolymer solids is then diluted in a suitable wetting agent, suchas isopropyl alcohol or acetone. The dispersion of oleophobicfluoropolymer solids is diluted at a ratio of wetting agent to thedispersion in the range of about 1:5 to 20:1, and preferably 1.8:1 to5:1. The diluted and stabilized dispersion 80 can be applied to themembrane 16 by any suitable conventional method, for example, byroll-coating, immersion (dipping), spraying, or the like. The dilutedand stabilized dispersion 80 impregnates the membrane 16, wets thesurfaces of the nodes 22 and fibrils 24 that define the pores 26 and thesurfaces that define the major sides 18, 20.

The dispersion prior to dilution has a surface tension γ_(LA) andrelative contact angle Ø so it cannot wet the pores 26 in the membrane16. The diluted and stabilized dispersion 80 has a surface tensionγ_(LA) and relative contact angle Ø so the diluted and stabilizeddispersion can wet all surfaces of the membrane 16. As the membrane 16is immersed in the diluted and stabilized dispersion 80, surfaces of themembrane 16 that define the pores 26 are engaged, wetted and coated bythe diluted and stabilized dispersion. The stabilizing agent has beenshown to extend the time that the solids in the dispersion take toagglomerate or settle out from just a couple of hours to in the range offour to eight hours or more.

The wetted membrane 16 is directed out of the reservoir 66. A mechanism70, such as a pair of squeegees or doctor blades, engages opposite majorsides 18, 20 of the wetted membrane 16. The doctor blades of themechanism 70 spread the diluted and stabilized dispersion and removeexcess diluted and stabilized dispersion from the wetted membrane 16 tominimize the chance of blocking pores 26 in the membrane 16. Any othersuitable means for removing the excess diluted and stabilized dispersionmay be used, such as an air knife.

The wetted membrane 16 is then trained over rollers 82. The stabilizingand wetting agents and any other fugitive materials, such as the water,acetone and ethylene glycol in the preferred dispersion, is subsequentlyremoved by air drying or other drying methods. The wetting agenttypically evaporates by itself but the evaporation can be accelerated byapplying relatively low heat, for example at least to about 100° C.,when IPA is the wetting agent. Wetting agent vapor V the moves away fromthe wetted membrane 16. Removal of the stabilizing agent generally willrequire an affirmative step for drying, such as by the application ofheat.

The wetted membrane 16 is then directed to an oven with heat sources 84.It may be necessary or desirable to enclose or vent the reservoir 66 andheat sources 84 with a hood 86. The hood 86 may be vented to a desiredlocation through a conduit 102. The hood 86 removes and captures thevapor V, such as, fugitive wetting and stabilizing agents, from thewetted membrane 16 and directs the captured material to a location forstorage or disposal. The heat sources 84 could each have two heatingzones. The first zone would be a “drying zone” to apply relatively lowheat to the wetted membrane 16 for example 100° C., to remove orevaporate any fugitive wetting and stabilizing agents that have notevaporated yet. The second zone would be a “curing zone” to coalesce theoleophobic fluoropolymer solids.

The heat sources 84 apply at least 200° C. heat for at least ten (10)seconds to the wetted membrane 16. Preferably, the heat sources 84 applyheat in the range of 220° C. to 240° C. for about thirty (30) seconds tothe wetted membrane 16. This amount heat permits the oleophobicfluoropolymer solids to reduce their surface tension to flow andspontaneously wet and better coat the surfaces defining the nodes 22 andfibrils 24. The oleophobic fluoropolymer solids flow and coalesce aroundthe surfaces of the nodes 22 and fibrils 24 to render the compositemembrane 12 oil and contaminating agent resistant. The amount andduration that the heat is applied to the treated membrane 16 allows theoleophobic fluoropolymer solids to coalesce orient and so tails made ofthe fluorocarbon side chains (not shown) extend in a direction away fromthe surfaces of the nodes 22 and fibrils 24 that are coated. Thecomposite membrane 12 exits the heat sources 84 and is then trained overrollers 104 and directed onto a take up reel 106.

A scanning electron microscope (SEM) photograph of an uncoated membrane16 is illustrated in FIG. 6. For comparison purposes, a compositemembrane 12 embodying the present invention is illustrated in FIG. 7.The composite membrane 12 includes the same uncoated membrane 16,illustrated in FIG. 6, with the coating 28 applied. The membranes 16(FIG. 6) and 12 (FIG. 7) are from the same production run. The SEMs areat the same magnification and it will be seen that the coated fibrils 24have a thicker appearance due to the layer of coating 28 on the fibrilsbut the pores 26 in the membrane 12 are not completely blocked. It willbe apparent that some pores 26 in the composite membrane 12 could beblocked, but such blockage is minimal and dependent on variables in thecoating process and structure of the membrane 16.

The composite membrane 12 embodying the present invention can be used infilters, outerwear garments apparel, tents, sleeping bags, protectivegarments, clean room garments, surgical drapes, surgical gowns and othertypes of barrier wear. The composite membrane 12 may be laminated orlayered with other porous materials or fabrics, such as woven cloth,non-woven fabric such as non-woven scrim, or foam material. The use ofsuch additional materials preferably should not significantly affect thewind and liquid penetration resistance, moisture vapor transmission orair permeability of the laminated fabric 10. The coating 28 is flexibleand durable so the composite membrane 12 is quiet, comfortable, washdurable and has good “hand”.

It is important that the composite membrane 12 remains air permeableafter the oleophobic fluoropolymer solids coalesce to form the coating28. Depending on the material, pore size, pore volume, thickness, etc.,of the porous membrane 16, some experimentation may be required tooptimize the process for applying the coating 28. The experimentationcan address the diluted and stabilized dispersion 80 with respect tosolids concentration, wetting agent selected, stabilizing agentselected, etc., in order to obtain an oil and water repellent coating 28that minimally influences air-permeability, yet provides the desiredlevel of oil and water repellency. The experimentation can also involveother methods of applying the diluted and stabilized dispersion,removing the wetting and stabilizing agents and coalescing theoleophobic fluoropolymer solids.

TEST DESCRIPTIONS

Moisture Vapor Transmission Rate

Moisture vapor transmission rates (MVTR) are preferably measured by aknown method termed the Dry Modified Desiccant Method (MDM). This methodprovides a high relative humidity in contact with the sample withoutdirect liquid contact with the sample membrane.

In the MDM method an expanded PTFE control membrane is tightly mountedin an embroidery hoop and floated upon the surface of a controlledtemperature circulating water bath. A desired amount of a desiccant isplaced into a cup. Another expanded PTFE control membrane is sealed tothe cup to create a tight and leak-proof microporous barrier containingthe desiccant. The test apparatus is located in an environmentallycontrolled room and the water is maintained at a predeterminedtemperature.

A membrane sample to be tested is mounted tight in another embroideryhoop and placed in the center of the control membrane in first hoop.After allowing the control membrane in the first hoop to equilibratewith the water for a predetermined time, the cup assembly is weighed tothe nearest {fraction (1/1,000)} gram and placed in an inverted manneron the center of the sample membrane in the second hoop.

Water transport is provided by the driving force between the water andthe desiccant providing water vapor movement in a direction from thewater bath to the desiccant. The sample membrane is tested for ameasured time and then the cup assembly is removed and weighed again towithin {fraction (1/1,000)} gram. The MVTR of the sample is calculatedfrom the weight gain of the cup assembly and is expressed in grams ofwater per square meter of sample surface area per 24 hours.

Moisture vapor transmission rates (MVTR) may alternatively be measuredusing relatively new test equipment and procedures defined herein as“dynamic test methods for air permeable materials”. One such dynamictest method is proposed ASTM Standard Test Method for Water VaporTransmission Rates of 500 to 100,000 g/m²/day through nonwoven andplastic barriers. Another such dynamic test method is described inNatick Technical Report (Natick/TR-98/014) entitled Convection/DiffusionTest Method for Porous Materials using the Dynamic Moisture PermeationCell.

Wetting Test

A challenge liquid, such as water, is sprayed or dropped onto thesurface of a sample of test material to visually assess the wet stateand the extent of infiltration of the liquid into the material. Whenwetted and penetrated by the test liquid, the samples generally changein appearance from opaque or semi-transparent to transparent.

Other test liquids that were used include 30, 40, 50, 60, 70, 80, 90 and100 wt-% isopropyl alcohol (IPA) in tap water.

Oil Penetration Test

A challenge oil is dropped onto the surface of a sample of test materialto visually assess the wetting of the liquid into the material. Whenwetted by the test oil, the samples generally change in appearance fromopaque or semi-transparent to transparent. The number reported is thatof the highest test oil number, having the lowest surface tension γ_(LA)value, that did not wet the test specimen.

Test oils with numbers 1-8, as described in the AATCC Technical Manualwere used.

Laundering Test

Test samples were placed in a test washing machine per AATCC 135 normalcotton cycle. The test samples are then removed from the washingmachine, thoroughly rinsed with water to remove the detergent solution,and air-dried.

After drying, the test piece is tested for wetting by application ofdrops of isopropyl alcohol (IPA) to the surfaces of the test piecerepresentative of both the inner and outer surfaces of the folded piece.The visual observations of the wetting test are reported below.

Air Permeability Test

Air permeability is measured by a Frazier Air Permeability Tester perASTM D737 or on a Textest FX 3300 Air Permeability Tester.

Without intending to limit the scope of the invention, the followingexamples demonstrate how the present invention may be practiced. Testresults are provided below to demonstrate the experiments performed andthe methodology used to direct the present invention.

MEMBRANE EXAMPLE 1

A microporous membrane 16 (manufactured by BHA Technologies, Inc. anddesignated QM011) made from ePTFE material was used. The membrane 16preferably has an average pore size in the range of about 0.35 to 1.0micron. The membrane 16 is preferably about 0.0015 inch thick. Themembrane 16 is preferably at least partially sintered.

TREATMENT EXAMPLE 1

The membrane 16 described above was treated with a diluted andstabilized Zonyl® 7040 dispersion. The following diluted and stabilizeddispersion was used for treatment:

treatment component percent by weight oleophobic fluoropolymerdispersion (Zonyl ® 7040) 13.9 stabilizing agent (water) 21.5 wetting ordiluting agent (IPA) 64.6

The samples were heated to between 220° C. and 240° C. for thirtyseconds to coalesce the solids onto the nodes and fibrils of the treatedmembranes. Over three hundred treated membranes were tested. Most of thepores in the treated membranes were not “blinded” or closed off. All thetreated membranes displayed air permeabilities of more than 0.1 CFM/ft²and most were in the range of 0.2 and 1.3 CFM/ft². All the treatedmembranes displayed an MVTR of more than 69,000 gr/m²/day and most werein the range of 70,000 and 102,000 gr/m²/day. The treated membraneswould hold out at least an 80% IPA challenge.

It is important to remember that comfort of the user of the compositemembrane 12 is the prime test criteria and is difficult to quantify.However, it has been found that due to the increased air permeability ofthe composite membrane 12 according to the present invention usercomfort is greater than previously known for an oleophobic, moisturevapor transmissive, wind and liquid penetration resistant membrane.

From the above description of preferred embodiments of the invention,those skilled in the art will perceive improvements, changes andmodifications. Such improvements, changes and modifications within theskill of the art are intended to be covered by the appended claims.

Having described at least one preferred embodiment of the invention, what is claimed is:
 1. A method of treating a membrane, said method comprising the steps of: providing a membrane with surfaces that define a plurality of pores extending through the membrane; providing a water-based dispersion of oleophobic fluoropolymer solids; stabilizing the dispersion with a stabilizing agent; diluting the dispersion with a water-miscible wetting agent; wetting surfaces which define the pores in the membrane with the diluted and stabilized dispersion; removing the wetting agent and the stabilizing agent from the membrane; and coalescing the oleophobic fluoropolymer solids of the dispersion on surfaces that define pores in the membrane.
 2. The method of claim 1 wherein said step of providing a membrane comprises providing a membrane with an average pore size of 10 microns or less.
 3. The method of claim 1 wherein said step of providing a membrane comprises providing a membrane made from expanded polytetrafluoroethylene.
 4. The method of claim 1 wherein said diluting step comprises diluting the stabilized dispersion in a wetting agent so the diluted and stabilized dispersion has a surface tension and contact angle relative to the membrane such that the diluted and stabilized dispersion is capable of wetting surfaces defining the pores in the membrane.
 5. The method of claim 1 wherein said step of stabilizing the dispersion comprises providing the stabilizing agent in a weight amount in the range of 0.5 to 5 times the weight of the wetting agent.
 6. The method of claim 1 wherein said step of stabilizing the dispersion includes providing water as the stabilizing agent.
 7. The method of claim 1 wherein said step of providing a dispersion of oleophobic fluoropolymer solids comprises providing a dispersion of acrylic based polymer with fluorocarbon side chains.
 8. The method of claim 1 wherein said step of diluting the dispersion of oleophobic fluoropolymer solids with a wetting agent comprises providing the wetting agent in a weight amount in the range of 2 to 10 times the weight of the dispersion.
 9. The method of claim 1 wherein said coalescing step comprises heating the oleophobic fluoropolymer solids to a temperature in the range of 200° C. to 240° C. for at least ten seconds to flow and coalesce the oleophobic fluoropolymer solids on surfaces defining the pores in the membrane without completely blocking the pores.
 10. A method of treating a membrane, said method comprising the steps of: providing a membrane with a node and fibril structure of which surfaces of the nodes and fibrils define a plurality of pores extending through the membrane; providing a water-based dispersion of oleophobic fluoropolymer solids, the dispersion has surface tension and contact angle properties relative to the membrane such that the dispersion is incapable of wetting pores in the membrane; stabilizing the dispersion with a stabilizing element; diluting the dispersion with a water-miscible wetting agent to provide a diluted and stabilized dispersion that has surface tension and contact angle properties relative to the membrane such that the diluted and stabilized dispersion is capable of wetting pores in the membrane; wetting surfaces of the nodes and fibrils of the membrane with the diluted and stabilized dispersion; removing the wetting agent and the stabilizing agent from the membrane; and coalescing the oleophobic fluoropolymer solids onto surfaces of the nodes and fibrils without completely blocking pores in the membrane by heating the wetted membrane.
 11. The method of claim 10 wherein said step of providing a membrane comprises providing a microporous membrane.
 12. The method of claim 10 wherein said step of providing a membrane includes the step of providing a membrane made of expanded polytetrafluoroethylene.
 13. The method of claim 10 wherein said step of stabilizing the dispersion comprises providing a weight amount of stabilizing agent in the range of 0.5 to 5 times the weight of the dispersion.
 14. The method of claim 10 wherein said step of stabilizing the dispersion includes providing water as the stabilizing agent.
 15. The method of claim 10 wherein said step of providing a dispersion of oleophobic fluoropolymer solids comprises providing a dispersion of acrylic based polymer with fluorocarbon side chains.
 16. The method of claim 10 wherein said step of diluting the dispersion of oleophobic fluoropolymer solids with a wetting agent comprises providing the wetting agent in a weight amount in the range of 2 to 10 times the weight of the dispersion.
 17. The method of claim 10 wherein said coalescing step comprises heating the oleophobic fluoropolymer solids to a temperature in the range of 200° C. to 240° C. for at least ten seconds to flow and coalesce the oleophobic fluoropolymer solids on surfaces defining the pores in the membrane without completely blocking the pores.
 18. The method of claim 10 wherein the average particle size of the oleophobic fluoropolymer is greater than about 0.1 microns.
 19. The method of claim 10 wherein the average particle size of the oleophobic fluoropolymer is about 0.15 microns. 